TUESDAY, March 15, 2016 (HealthDay News) -- A look at recent measles outbreaks in the United States finds more than half of the cases involved unvaccinated children.

Vaccine refusal was also often the culprit in whooping cough (pertussis) outbreaks as well, according to the new study.

"When vaccine refusal rates are high, the rates of measles and pertussis are higher," said researcher Dr. Varun Phadke, a fellow in infectious diseases at Emory University in Atlanta.

In 2000, the United States was declared measles-free. But infected travelers returning to the country have caused outbreaks, and unvaccinated children are most at risk, research has shown.

This study found that among more than 1,400 cases since 2000, more than half of patients had no history of vaccination. Most had nonmedical exemptions -- based on religious or philosophical beliefs -- not medical exemptions, researchers said.

The report was published March 15 in the Journal of the American Medical Association.

Measles and whooping cough are highly contagious, and when fewer people are vaccinated, widespread protection known as herd immunity breaks down, giving these viruses free rein to spread.

Dr. Matthew Davis is a professor of pediatrics and communicable diseases at the University of Michigan and author of an accompanying journal editorial. "Vaccination is one of the most effective and safest tools to protect our population against many diseases," he said.

But rates of nonmedical vaccine exemptions have increased steadily over the past 20 years, the researchers said in background notes.

Davis and Phadke agreed that states need to make it harder to obtain vaccine exemptions.

In California, where a 2014 measles outbreak led to 111 cases, legislation was passed to eliminate nonmedical exemptions to vaccination, Phadke said. He'd like to see other states follow California's lead.

"States that make it harder not to get vaccinated have lower rates of vaccine-preventable diseases," Phadke said. "Strategies aimed at decreasing vaccine refusal may have an important impact on the community."

To determine how many outbreaks since 2000 were the results of vaccine refusal, Phadke and colleagues reviewed 18 previously published studies that included 1,416 measles cases. Among these patients, 57 percent had not been vaccinated, they found.

Looking at nearly 1,000 cases that had detailed vaccination data, they found nearly 600 were not vaccinated despite being eligible. In those cases, 71 percent had refused vaccination for religious or philosophical reasons, Phadke's team found. The rest had medical reasons for not being vaccinated.

The researchers also looked at 32 reports on whooping cough outbreaks, which involved more than 10,600 patients. In the five largest outbreaks, up to 45 percent of the patients had not been vaccinated or had not had booster shots, the study authors said.

However, several of these outbreaks occurred in highly vaccinated populations. This indicates that immunity to whooping cough was waning, Phadke said.

The researchers also found that in eight of 12 whooping cough outbreaks for which detailed vaccination data was available, as many as 93 percent of unvaccinated patients were intentionally not immunized.

This trend is worrisome, Phadke said. "We are going to see more measles outbreaks," he predicted.

"Outbreaks of diseases like measles and pertussis remind us that there are still ways to improve how we use vaccines to safeguard the health of children and adults across the United States," he said.

Because some vaccines are less effective than others -- the whooping cough vaccine, for example -- more booster shots may be needed along with a new, more effective vaccine, Davis said.

Also, it's not enough to just vaccinate children, he said. Outbreaks can be prevented "by doing a better job of vaccinating adults and not just trying to do the best job of vaccinating children," Davis added.